1. Field of the Invention
The present invention relates to a metal mask, and more particularly, to a mask frame assembly used in deposition of thin films of an organic electroluminescent device.
2. Description of the Related Art
Recently, electroluminescent (EL) devices regarded as self-luminous type display devices have been receiving a lot of attention as a next-generation display device due to the advantages of a wide viewing angle and good contrast and rapid response characteristics.
EL devices are classified into inorganic EL devices and organic EL devices depending upon the material of an emissive layer. Organic EL devices can realize color display and have better luminance and response characteristics than inorganic EL devices.
An organic EL device includes a series of first electrodes formed as a predetermined pattern on a transparent insulating substrate, an organic emissive layer formed on the transparent insulating substrate by vacuum deposition, and a series of second electrodes formed on the organic emissive layer to act as a cathode electrode intersecting the first electrode layers.
In the manufacture of an organic EL device having such a structure as described above, the first electrodes are formed of indium thin oxide (ITO). In patterning an ITO layer into the first electrodes, the ITO layer is wet etched in a FeCl2 containing etchant. However, when this photolithography method is applied to etch the second electrodes, liquid permeates into the interface between the organic emissive layer and the second electrodes while the resist used is stripped off and the second electrodes are etched, thereby degrading the performance and lifetime characteristics of the organic EL device.
To address these problems, methods of deposition of an organic luminescent material used as an organic emissive layer and second electrodes have been suggested.
In the manufacture of an organic EL device using these methods, first electrodes of ITO are formed on a transparent insulating substrate as a striped pattern by photolithography. An organic emissive layer is deposited on the transparent insulating substrate where the first electrodes have been formed, and a mask pattern that matches a desired pattern of second electrodes is placed on the organic emissive layer, and a second electrode material is deposited on the transparent substrate.
Korea Laid-open Patent Publication No. 2000-60589 discloses a mask used in deposition of an organic emissive layer or a cathode (second) electrode, an organic EL device manufactured using the mask, and a method of manufacturing the organic EL device. This mask includes a series of long slits spaced a predetermined distance apart in its main thin plate.
Korea Laid-open Patent Publication No. 1998-71583 discloses a mask of metal thin plate having a slit portion and a bridge portion in a mesh pattern.
Japanese Laid-open Patent Publication No. 2000-12238 discloses a mask having an electrode mask portion and a pair of electrode pad mask portions. The electrode mask portion includes a masking portion having a plurality of strips arranged parallel to each other with a width, which is substantially equal to a cathode (second) electrode gap, and a connecting portion which connects the strips at their both ends.
The conventional masks described above include long striped slits in the metal thin plate. Therefore, although the edges of the metal thin plate are supported under tension by a frame, the long slits sag away from a substrate by the weight of the mask. This problem becomes serious as the size of the substrate increases. In addition, thermal expansion of the mask during deposition of the cathode facilitates the sagging of the slits by their weight.
An example of a mask applied to produce organic EL devices on a large scale is illustrated in FIG. 1. The mask of FIG. 1 includes a number of unit masking pattern portions 12 for each organic EL device substrate in a thin metal plate 11 and is supported under tension by a frame 20.
This conventional mask 10 has a relatively large size for large scale production, so the problem of sagging by its own weight becomes serious even when uniform tension is applied to each side of the mask to fix it to the rectangular frame 20. In welding the large-sized thin metal plate to the frame 20, the width of each slit 12a of the unit masking pattern portions 12 should be maintained within a predetermined tolerance range. As tension is applied to each side of the mask 10 to prevent the problem of strip sagging, the pitch of the slit in each unit masking pattern portion 12 is distorted beyond a predetermined tolerance range. In particular, as the slits of a particular unit masking pattern portion of the mask 10 are distorted, the force of the deformation is transferred to the slits of neighboring unit masking pattern portions and thus distorts the neighboring slits. The result is a shadow effect of the mask 10 in deposition of an organic layer or cathode so that the resulting organic layer or cathode pattern is beyond a predetermined tolerance range. This shadow effect is more serious in the traverse direction of the slits 12a of the mask 10.
The distortion of each unit masking pattern portion 12 increases variations in total pitch due to the displacement of unit electrode patterns on the substrate with respect to the original pattern of each unit masking pattern portion 12, thereby disabling accurate deposition of red, blue, and green organic layers on the separate unit electrode patterns of the substrate. In addition, pitch and total pitch adjustment of the unit masking pattern portions formed in a large-sized thin metal plate is possible in only a restricted area, which limits the size of the mask 10.
When tension is applied to each side of a single mask 10 to fix it to a frame 20, side support bars 21 of the frame 20 are curved inwards and upper and lower support bars 22 of the frame 20 are curved outwards by the tension, as shown in FIG. 2. Alternatively, as illustrated in FIG. 3, the side support bars 21 are curved outwards and the upper and lower support bars 22 of the frame 20 are curved inwards by the tension.
Although uniform tension is applied to weld the mask 10 to the frame, the distortion of the mask and displacement of the unit electrode patterns formed on a substrate to the mask patterns make total pitch adjustment difficult.
Japanese Laid-open Patent Publication No. 2001-247961 discloses a mask to eliminate the creeping of strips due to thermal expansion of the mask, which are separated by slits. This mask is used in formation of a patterned layer on a substrate by deposition and includes a mask portion with a number of partitions defining separate first openings and a screen portion with a number of second openings of a smaller size than the first openings. The screen portion includes a magnetic material and is arranged on the mask portion such that the second openings are aligned over the respective first openings.
Japanese Laid-open Patent Publication No. 2001-273979 discloses the structure of a magnetic mask. Japanese Laid-open Patent Publication No. 2001-254169 discloses a deposition mask having an affinity for a target substrate and having a fine pattern portion supported by ribs.
Although the conventional masks described above are supported by the frame and are formed of a magnetic material to give a magnetic affinity for a target substrate, the inherent problems, such as variations in strip pitch due to the weight and the tensile strain of the mask, damage of the organic layer in moving and handling the mask due to increased attraction to the glass substrate, and total pitch variations due to internal stress of the mask and frame occur.